In the field of forensic ballistics, microscopic tool marks present on different types of objects are compared to find matching tool marks to link the objects together, such as bullets that may eventually be linked to a recovered firearm. Comparisons are typically done using comparison microscopes, where an expert will visually compare the tool marks found on the two objects and use his experience to determine a match.
Some software tools exist to assist the expert in making the determination. For example, a 3D acquisition sensor can be used to acquire the topography, and possibly a 2D image, of an object with tool marks. Visual analysis of the resulting data can then be realized via a virtual comparison microscope which displays the acquired data.
A virtual comparison microscope is a software tool that displays the topography of one or several objects which may have tool marks. Its input is a list of virtual objects, each defined by a cloud of points in a 3D coordinate system which are connected in order to generate polygonal faces over the whole surface of the object. The minimal requirements of the virtual comparison microscope is the ability to display such clouds of points by using rendering techniques, move the virtual objects in space in translation (X, Y, translations) and rotation about any axis, zoom on the object, simulate different types of lighting conditions. The light source can be translated and rotated as well. Some virtual microscopes can also superpose a relevant texture on the virtual object. A virtual comparison microscope has some unique properties which are never met on a physical comparison microscope: an infinite depth of field, which removes any focus problems, and optimal design of the light source and reflectivity models, which brings a better control on the number of very bright or dark areas. However, the software tool is not used to make the final call. The expert must confirm potential matches using the comparison microscope.
A physical microscope has several limitations: finite depth of focus, non correspondence between the symmetry (or rotation) axis of the object, if any, and the rotation axis of the motor displacing the object, and independent rotation axes and light sources when two objects are analyzed at the same time.
Visual comparison of microscopic tool marks present on two objects under a comparison microscope may be significantly challenging, especially when only a few marks are present on the objects. In this case, the number of matching marks on any single region of interest may not be sufficient to state that the pair of analyzed objects is a “conclusive pair”, i.e. a pair of objects which was in contact with the same tool or was fired with the same firearm. The tool mark or firearm expert must then look for groups of matching marks over several regions of interest.
This operation may be challenging due to limitations of the comparison microscope. This is especially true for bullets. In some instances, the bullets may have a circular cross section, i.e. a pristine form, and the symmetry axis of each bullet may coincide with the rotation axis when placed in the comparison microscope. In this case, the expert can compare the bullets by manually rotating each bullet around its corresponding rotation axis. However, in other instances, the symmetry axis and the rotation axis for one bullet may not coincide. The firearm expert must then apply several manual operations comprising rotating the bullets, translating the bullets and varying the position and intensity of the light sources in order to compare the two bullets. These operations are particularly challenging for bullets with very few small marks.
A further limitation of the physical comparison microscope is its finite depth of focus. When the topography in the field of view is not locally flat, it is not possible to analyze this whole area view with optical focus.
With such limitations, it is challenging and time consuming to look for matching marks between two objects with tool marks.
There is a need to improve the tools presently available to experts to facilitate the comparison that must be done using the comparison microscope.